Differences in baseline and drug-induced frequencies of somatic and germ cells have been demonstrated. This capability of the SCE test was utilized to determine the relative sensitivity of various mouse tissues to the carcinogen ethyl carbamate. 5-Bromodeoxyuridine (BrdU)-dye methodology was utilized for examining SCE in vivo and in vitro. The extent of SCE induction in the tissues used correlates well with their known sensitivity to this agent with respect to the induction of carcinogenic and mutagenic endpoints.